1. Field of the Invention
The present invention relates to blends of unsaturated polyester resins, and in particular to blends of RLP-modified unsaturated polyester base resins with reactive liquid polymers. More particularly, the invention relates to the modification of unsaturated polyester base resins, which already have reactive liquid polymers reacted into their backbones, by admixing or blending reactive liquid polymer additives with the prereacted unsaturated polyester resins and curing the resins using a free radical source. The invention further relates to the above resin in a cured form, a shaped article made from the resin and a method for making the blended resin composition.
2. Background Information
Heretofore, in general, there have been two known methods for modifying or toughening thermoset resins with reactive liquid polymers (hereinafter RLPs) to improve properties such as fracture energy (G.sub.Ic) and impact strength without adversely affecting the thermomechanical properties of the resins. The first prior art method is the adduction or reaction of reactive liquid polymers into the backbone of the thermoset resins. U.S. Pat. No. 4,290,939 relates to coreacting a toughening agent with components of an unsaturated polyester resin in order to introduce the toughening agent into the backbone of the unsaturated polyester resin and thus produce a toughened stable resin. Suitable toughening agents are carboxyl and hydroxyl terminated reactive liquid polymers which are used in an amount of 1 to 30, and preferably 1 to 15 parts by weight per 100 parts by weight of reactant monomers used to prepare the polyester resin. Such adducted systems can be cured in a conventional manner to give elastomer or rubber-modified thermoset resin systems having a significant enhancement in toughness, as measured by fracture energy, over their unmodified counterparts. However, in general, it has been discovered that with unsaturated polyester resins only a finite amount of RLP can be reacted into the backbone of the resin. Attempting to incorporate large amounts of RLP undesirably causes the polyester cook to gel.
A second prior art method of modifying thermoset resins by admixing a RLP with the resin, allows one to incorporate higher levels of rubber, but the polymer additive tends to have limited miscibility with the unsaturated polyester resin. Such a general lack of miscibility between RLPs and unsaturated polyester resins results in the necessity to thoroughly mix the composition prior to use, which is inconvenient and impractical for most applications.
It has now been discovered that by combining the two methods, that is, admixing RLP with an unsaturated polyester resin having RLP reacted into its backbone, higher reactive polymer levels within the unsaturated polyester resin with improved miscibility can be achieved. Moreover, such polymers have comparable or better toughness when compared to polymers obtained by the previous methods at the same elastomer concentrations. Moreover, surprisingly, it has been discovered that significant improvement in toughness (fracture energy) of the inventive unsaturated polyester resins is achieved when the total rubber content is at least 10 phr.